In order to meet Federal Aviation Regulations and satisfy safety concerns, present-day transport category aircraft are generally required to have robust flight control systems with redundant control paths. In order to provide redundancy in the flight control systems, manufacturers often utilize multiple control paths in both reversible and irreversible flight control systems, including fly-by-wire and fly-by-light flight control or augmentation systems.
Providing multiple control paths for each control axis often adds complexity, weight, and increased friction to an aircraft flight control system. This is true even for fly-by-wire flight control systems because they typically include mechanical components. The increased weight and friction due to multiple control paths can significantly increase the control system friction and breakout forces, i.e., the forces a pilot must exert to cause an initial movement of a flight control surface away from the trimmed or neutral position, including control centering forces. For example, the pilot must apply at least a threshold level of force to turn the yoke of a control system to the left or right before any lateral flight control surface begins to move. Accordingly, a “dead band” or “friction and breakout band” encompasses the range of forces between the left and right threshold levels. The magnitude of the friction and breakout forces can affect a pilot's ability to make small precise inputs around the trim point and can decrease aircraft predictability.
FIG. 1 schematically illustrates a typical transport category aircraft lateral flight control system in accordance with the prior art. The control system has two redundant control loops. The pilot's control loop 1 includes a pilot's yoke 6 positioned to receive inputs from the pilot, a left cable loop 3, and other elements coupled to transmit pilot inputs from the pilot's yoke 6 to a left flight control surface 5a. The first officer's control loop 2 includes a first officer's yoke 7 positioned to receive inputs from the first officer, a right cable loop 4, and other elements coupled to transmit first officer inputs from the first officer's yoke 7 to a right flight control surface 5b. Both control loops 1, 2 are linked together by a forward connector 8 and an aft connector 9, so that when the pilot or the first officer makes a control input, (a) the control input is transmitted through both control loops 1, 2, and (b) both flight control surfaces 5a, 5b can be actuated. If the left cable loop 3 or the right cable loop 4 fails, the remaining cable loop provides continual lateral control of the aircraft by transmitting control inputs from either yoke 6, 7 to both control surfaces 5a, 5b. During normal operation, the left cable loop 3 and the right cable loop 4 both move in response to each aircrew input, and the friction and weight of both cable loops contribute to the friction and breakout force in the aircraft lateral flight control system.